New Mexico Mineral Symposium — Abstracts

The Eocene Wind Mountain laccolith crops out over approxi-mately 2 km² in southern Otero County, New Mexico. It is one of several small intrusions that were emplaced as discordant sheets, sills, and laccoliths into Permian and Cretaceous sediments of the Cornudas Mountains area. It consists of an analcime nepheline syenite. Miarolitic cavities in the laccolith contain a suite of uncommon minerals that is similar to the suite found at Mont St.-Hilaire, Quebec, Canada. Most notable is george¬chaoite, NaKZrSi₃O₉•3H₂0, a new mineral related to gaidonnayite, Na₂ZrSi₃O₉•3H₂0. Georgechaoite occurs as white, twinned, ortho¬rhombic crystals up to 1 mm in size. It is associated with microcline, acmite, nepheline, analcime, catapleiite, monazite, and a Mn-rich chlorite. Other minerals not found directly asso¬ciated with georgechaoite include chabazite, eudialyte, calcite, thomsonite, and natrolite. It is likely that many other minerals will be found by collectors at Wind Mountain, as well as at some of the other intrusions in the Cornudas Mountains area.

The miarolitic cavities range in size from approximately 1 cm to 3 cm in diameter. The crystals in the cavities are usually small, seldom exceeding 5 mm (for minerals such as microcline, nepheline, and acmite) and commonly only I mm to 2 mm for the rarer minerals. The sequence of formation of the minerals in the cavities is as follows (earliest to latest): microcline, nepheline, analcime, acmite, chlorite, catapleiite, monazite, and georgechaoite. A brief description of some of the species follows.

Analcime has formed in part from the alteration of nepheline and is commonly found as coatings of euhedral crystals replacing nepheline crystals.

Catapleiite is found as small (<1 mm), euhedral, orange to white, hexagonal, tabular crystals. They commonly form rosette-like groups and are perched on microcline or acmite.

Georgechaoite occurs as small (<1 mm), white, twinned, prismatic crystals. They are commonly found growing on either microcline or acmite.

Monazite occurs as small (<1 mm), yellow, prismatic crystals that are commonly perched on acmite.

Nepheline occurs as hexagonal prisms commonly altered, in part, to analcime.

Thomsonite is found as radiating balls of transparent prismatic crystals.